FKBP12 is a small immunophilin that binds with high affinity to sarcoplasmic reticulum Ca2+ release channels (ryanodine receptors, RyRs) and transforming growth factor 1 receptors (TI RI). In this application we are going to test the hypothesis that the gain of E-C coupling is regulated in a biphasic manner by FKBP concentration. Specifically we will: 1) Demonstrate that FKBP12 binding to RyR1 controls the gain of E-C coupling in a biphasic manner, thereby, modulating Ca2+ stores, force production, fatigue, and recovery from injury; 2) Evaluate the ability TRI activation to increase the gain of E-C coupling via FKBP12; 3) Evaluate the ability of drugs that decrease FKBP12 binding to RyR1 to slow the development of fatigue and enhance recovery from injury and 4) Identify the amino acids on RyR1 that contribute to FKBP12 binding. The research described in this application will clarify the role of FKBP12 in skeletal muscle function and lay the groundwork for the development of new therapeutic interventions to slow diaphragm fatigue and enhance recovery from injury.